While it is known that sensorless control of rotary electrodynamic machines is possible, a number of novel linear machines and relatively new applications for the same have motivated the need for robust and cost-effective control methods.
One such new application where novel and effective control methods for linear machines is desired relates to thermoacoustic devices such as thermoacoustic refrigerators or similar applications where it may be desirable to track an acoustic resonance of the refrigerator for maintaining an optimum operating condition at the reciprocating linear electrodynamic machine.
It is appreciated that many thermoacoustic devices are designed to be co-resonant with the linear machine that drives the device. In other words, the thermoacoustic device is designed to perform optimally when the mechanical resonant frequency of the linear machine is equal to the resonant frequency of the thermoacoustic device. In practice, the acoustic resonance can be a function of the operating temperatures or the cooling load, and it is then desirable for the excitation provided to the machine, to be adjusted in a manner to re-optimize performance. Thus, knowledge of this functional relationship between the drive machine and the load device is required for one to control the performance of the combined system by controlling the excitation administered to the drive machine.
In conventional thermoacoustic refrigerators, for example, acoustic pressure and actuator displacement at the linear machine are typically monitored while a nominally harmonic drive frequency and amplitude (either voltage or current) is adjusted to achieve a desired operating point in view of these parameters. This desired operating point may illustratively correspond to an acoustic resonance at the load, maximum electroacoustic conversion efficiency between the linear machine and the load, maximum power delivery to the load, or to achieve some other desirable operating point for the thermoacoustic system.
Most often, pressure and displacement are monitored directly using sensors. However, U.S. Pat. No. 5,342,176 issued to Redlich discloses a method for obtaining the piston displacement in a free piston compressor for a linear electrodynamic machine using only the measured terminal current and voltage of the machine. Redlich's objective for obtaining the piston displacement through the disclosed method was primarily concerned with the control of the amplitude of the actuator in the linear machine. Such control is necessary to avoid collision with stationary parts of the linear machine, but not as a precursory step for controlling a thermoacoustic load, since the application is for use with a compressor where efficient compression requires as small as possible a space between the piston and the end of the compression space. Further, in the absence of a pressure or force determination, it is not possible to determine conditions such as an acoustic resonance, the machine efficiency, or to adjust for maximum power transfer.
U.S. Pat. No. 6,289,680 issued to Oh et al. discloses a method of controlling a linear motor used in a compressor application wherein force and displacement of the linear motor is mentioned, however, there is no suggestion that force or pressure may be extracted from the linear motor in a completely sensorless manner, and these dynamical parameters be used to provide a more robust and cost-effective control means.
Accordingly, there is a need for a method of sensorless control of a linear reciprocating electrodynamic machine that obviates the need for pressure and displacement transducers traditionally associated with the control of such machines that are commonly used for driving thermoacoustic refrigerators or similar frequency dependent loads.